This invention relates to a tunable electrooptic add-drop filter apparatus and method. In particular, the invention relates to a tunable electrooptic add-drop filter apparatus and method in filters fabricated on a birefringent electrooptic substrate whereby a narrow range of optical frequencies are added to an optical fiber and a narrow range of optical frequencies are dropped from a fiber while leaving other frequencies unaffected.
Wavelength division multiplexing is widely used in fiber optic communication to increase the data capacity of an optical fiber. Currently, 16, 32, or more data channels are transmitted in parallel on a single mode fiber using different optical carrier frequencies for each channel. To combine and separate these channels, a variety of frequency-selective components have been developed, including multilayer dielectric coatings, fiber Bragg gratings, arrayed waveguide gratings, and Mach-Zehnder chains. None of these techniques satisfies industry requirements for high-speed tunability and wide frequency tuning range.
An example of a prior-art electrooptic tunable filter (EOTF) for performing the add-drop function is illustrated in FIG. 1. Such a filter has been demonstrated previously by applicants at Texas AandM University in the substrate material lithium tantalate (LiTaO3) [1].
The substrate for the prior art tunable filter (FIG. 1) is a single-crystal of a birefringent electrooptic material such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The waveguides are single mode for both TE and TM polarizations. Between the two directional coupler polarizing beam splitters (PBS""s) is a spatially periodic dielectric film to produce polarization conversion, and electrodes for applying a tuning voltage.
Wavelength selectivity is determined by the phase-matching condition which governs coupling between the polarization states induced by the periodic film, as described by                               Δ          =                                                    2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                v                ⁢                                  xe2x80x83                                ⁢                                  (                                                            n                      1                                        -                                          n                      3                                                        )                                            c                        ±                                          2                ⁢                                  xe2x80x83                                ⁢                π                            Λ                                      ,                            (        1        )            
where xcex94 is the phase mismatch constant, xcexd is the optical frequency, n1 and n3 are the principal refractive indices of the birefringent substrate material, and xcex9 is the spatial period of the film. The frequency xcexdj for which maximum polarization conversion occurs corresponds to the phase-matching condition xcex94=0. For frequencies far from xcexdj, xcex94 is large and very little polarization conversion will take place.
Tuning is accomplished in this prior art device by applying a voltage to electrodes on the surface of the substrate. The resulting electric field in the waveguide region causes a change in the birefringence (n1xe2x88x92n3) via the linear electrooptic effect (Pockels effect). It follows from eq. 1 that a change in the birefringence causes a change in the frequency for which phase-matched polarization coupling occurs.
Waveguides can be fabricated by a process which involves (1) deposition of a thin (90 nm) layer of titanium on the surface of the substrate, (2) patterning the titanium by a process of photolithography and etching, and (3) diffusing the titanium into the substrate at 1050xc2x0 C. To produce the electrode pattern, a uniform aluminum film is deposited on the substrate, patterned photolithographically, and etched. Finally, in the case of the tunable filter, a silicon dioxide film is deposited at 350xc2x0 C., masked with photoresist at room temperature, and etched to produce a periodic bar pattern in the film. Strain resulting from the mismatch in thermal expansion coefficients between film and substrate causes polarization conversion in the waveguides.
Another prior art design which is the subject of the co-pending patent application Ser. No. 09/737,206 is illustrated in FIG. 2. This design is promotes ease of manufacturing for the EOTF by eliminating the need for PBS""s, which are difficult to produce with the required tolerances. The design of FIG. 2 differs from that of FIG. 1 at least in that the strain-inducing strips are offset by xcex9/2 in the top waveguide relative to the bottom one, and the optical path difference in the top waveguide differs from that in the bottom one by xcex/2.
An expanded view of a section of waveguide containing the strain pads and electrodes for electrooptic tuning is shown in FIG. 3. The diagram in FIG. 3 could represent either the upper or lower waveguide in the EOTH of either FIG. 1 or FIG. 2. The performance of the tunable prior art filters of FIG. 1 or FIG. 2 is determined by the dependence of polarization conversion on optical frequency in this waveguide section.
From eq. (1) it is determined that the spatial period xcex9 of the strain pads in the EOTF is determined by the center frequency xcexd* of the spectral region in which the filter is intended to operate and the refractive indices n1 and n3 of the birefringent substrate. The optical wavelength region of most interest for optical fiber communication is 1530-1560 nm. The frequency of the center of this wavelength regime is c/xcex*, with c the free-space speed of light and xcex*=1545 nm, is xcexd*=2.998xc3x97108/1545xc3x9710xe2x88x929=1.929xc3x971014 Hz. For a lithium niobate substrate, with n1=2.2118 and n3=2.1384, it is calculated from eq. (1) that xcex9=21.05 xcexcm.
A theoretical plot of the efficiency for polarization conversion X on optical frequency, measured relative to the frequency for maximum polarization conversion, is given in FIG. 4. The plot assumes that the substrate material is lithium niobate (LiNbO3), a total length for the polarization conversion region of 3.6 cm, and a uniform coupling constant induced by the strain pads of 0.139 cmxe2x88x921. The same conditions apply to the plot of FIG. 5, in which the frequency scale is expanded.
The length of the polarization conversion region was chosen to give the first nulls in the conversion spectrum at xc2x1100 GHz relative to the central peak where the conversion efficiency is a maximum. This would correspond to application in a wavelength-division-multiplexed (WDM) communication system in which the channel spacing corresponds to the International Telecommunication Union (ITU) specification of 100 GHz channel spacing. Thus, when a particular channel is selected, the adjacent channel would correspond to a null, to reduce crosstalk between channels.
The present invention addresses several deficiencies with the prior-art EOTF designs described by the diagrams in FIGS. 1-3 and the calculated response curves of FIGS. 4 and 5. Among these deficiencies are:
(1) The required tuning voltage is too high. To tune the center frequency of one of these prior art filters by 100 GHz is estimated to require about 7 V with an electrode spacing of 10 xcexcm. Thus, tuning over 32 channels would require a voltage swing of 32xc3x977=224 V, and to tune over 64 channels. requires a voltage swing of 448 V. Such large voltages applied over such a short distance causes degradation or even destruction of the EOTF.
(2) The nulls in the conversion spectra of FIGS. 4 and 5 are not equally spaced at the desired 100 GHz separation. This adversely affects the need to minimize crosstalk in a WDM system.
(3) The time delay experienced by the light in traversing the EOTF is different for the TE and TM polarizations due to the birefringence of the substrate. For example, for a substrate length Lsub of 7 cm, with n1xe2x88x92n3=0.0734, the delay difference is Lsub(n1xe2x88x92n3)/c, with c the free-space speed of light, which is calculated to be 7xc3x970.0734/2.998xc3x971010=17 ps. Such a delay leads to degradation of the xe2x80x9ceye diagramxe2x80x9d in a high-data-rate system, particularly at data rates of 10 Gb/s or higher.
(4) The length of the polarization conversion region required to achieve a 50 GHz channel spacing for WDM is not compatible with the size of available electrooptic substrates. The length of the conversion region is inversely proportional to the channel spacing, so that for a 50 GHz spacing the conversion region needs to be 3.6xc3x972=7.2 cm long. When the length required for beam splitters and separating waveguide regions are taken into account, a 50 GHz device could not be accommodated on the largest commercial lithium niobate substrates, which are 3xe2x80x3 (7.5 cm) in diameter.
Accordingly, the tunable electrooptic add-drop filter apparatus and method of the present invention includes, in filters fabricated on a birefringent electrooptic substrate, two input single mode waveguides. A first beam splitter is connected to the waveguides and a polarization converter is connected to each of the waveguides after the first beam splitters wherein each polarization converter includes more than one set of spaced apart, spatially periodic, strain-inducing pads. Electrodes are located in proximity to each of the polarization converters. A second beam splitter is connected to the waveguides after the polarization converter and two output single mode waveguides are connected to the second beam splitters.
In another aspect of the invention, the length of the polarization converter is given by the formula: Ltot=NcL1+(Ncxe2x88x921)L2 where: Nc=an integral number of polarization coupling regions of length L1 and L2=longer regions between the polarization coupling regions in which polarization coupling does not occur. In a further aspect, a multiplicity of individual strain-inducing pads are provided wherein the spacing between any two such strain-inducing pads is equal to an integer times a particular minimum spacing between adjacent strain-inducing pads. In another aspect of the invention, the widths of the strain-inducing pads are varied. In a further aspect of the invention, the polarization converter has a center and edges and the width of the strain-inducing pads is greater at the center of the polarization converter and tapers monotonically towards the edges.
In another aspect of the invention, polarization maintaining fibers are connected to each input and output single mode waveguide of the tunable electrooptic add-drop filter apparatus so as to compensate for the difference in time delay for the two polarizations of light propagating in and through the single mode optical waveguides in the birefringent electrooptic substrate. In a further aspect of the invention, multiple tunable electrooptic add-drop filters of the present invention are connected in series. In another aspect of the invention, the multiple electrooptic add-drop filters have different values of Nc where Nc is an integral number of polarization coupling regions.
In another embodiment of the invention, in filters fabricated on a birefringent electrooptic substrate, a tunable electrooptic add-drop filter apparatus includes two input/output single mode waveguides. A beam splitter is connected to the waveguides. A polarization converter is connected to each of the waveguides wherein the polarization converter includes more than one set of spaced apart, spatially periodic, strain-inducing pads. Electrodes are provided on the substrate in proximity to the polarization converter. Finally, a reflector is connected to the waveguides after the polarization converter.
In a further aspect of the invention, the length of the polarization converter is given by:. Ltot=NcL1+(Ncxe2x88x921)L2 where: Nc=an integral number of polarization coupling regions of length L1 and L2=longer regions between the polarization coupling regions in which polarization coupling does not occur. In a further aspect of the invention, a multiplicity of individual strain-inducing pads are provided wherein the spacing between any two such strain-inducing pads is equal to an integer times a particular minimum spacing between adjacent strain-inducing pads. In a further aspect of the invention, widths of the strain-inducing pads are varied.
In yet another aspect of the invention, the polarization converter has a center and edges and the width of the strain-inducing pads is greater at the center of the polarization converter and tapers monotonically toward the edges. In another aspect, a voltage tuner is connected to the electrode. In another aspect of the invention, polarization maintaining fibers are connected to each input/output single mode waveguide. In another aspect, an optical circulator is connected to each input/waveguide.
In another embodiment of the invention, in filters fabricated on a birefringent electrooptic substrate, a tunable electrooptic add-drop filter method is provided beginning with the step of providing two input single mode waveguides on the substrate. A first beam splitter is connected to the waveguides. A polarization converter is connected to the waveguides after the first beam splitter wherein the polarization converter is conformed to include more than one set of spaced apart, spatially periodic, strain-inducing pads. An electrode is connected to the polarization converter and a second beam splitter is connected to the waveguides after the polarization converter. Next, two output single mode waveguides are connected to the second beam splitter. Finally, a voltage tuner is connected to the electrode and voltage is applied to the electrode through the voltage tuner.
In a further aspect of the invention the step of forming the length of the polarization converter is provided in accordance with the formula: Ltot=NcL1+(Ncxe2x88x921)L2 where: Nc=an integral number of polarization coupling regions of length L1 and L2=longer regions between the polarization coupling regions in which polarization coupling does not occur.